It has already been proposed to undertake a checking operation of this type by causing a light beam to sweep in a zig-zag path across the transverse margin and measuring the quantity of light reflected by means of a photo-electric detector and an electronic multiplier. The quantity measured and the length of pulse corresponding to the width of the margin vary however as a function of the speed of movement of the support, such that any variation of this speed renders the measurement inaccurate.
It has also been proposed to use optic fibres and to count the number of optic fibres illuminated by the light reflected by the margin and to compare this number with a reference value. The power of resolution of such a device is however relatively low owing to the dimension of the optic fibres. Moreover, it is necessary to have one detector per optic fibre.
Finally, the two methods mentioned above do not provide a measurement of the parallelism of each of the margins with the edges of the support.